Device for superposing optical signals with different wavelengths

ABSTRACT

The invention relates to a device for superposing optical signals of different wavelengths. The device is provided with a light guide grid disposed between an input free beam coupler and an output free beam coupler, the input free beam coupler being provided at its input side with at least two input light guide units and an output light guide unit including a plurality of output light guides being to the output side of the free beam coupler. The aperture areas of the input light guide units are arranged and dimensioned relative to the the aperture areas of the output light guides such that each output light guide is affected by at least two optical signals of different wavelengths from the input light guides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for superposing optical signals ofdifferent wavelengths, the device being provided with an input free beamcoupler and an output free beam coupler the free beam sections of whichare coupled to each other by phase shift light guides of a light guidegrid, with a first input light guide unit connected to the input freebeam coupler and with at least one further input light guide unitwhereby at least one wavelength may be conducted in each input lightguide unit, and with an output light guide unit provided with aplurality of output light guides, the output light guides beingconnected to the output free beam coupler and arranged such that each ofthe output light guides may be energized by at least two optical signalsof different wavelengths.

2. The Prior Art

Such a device is known from U.S. Pat. No. 5,440,416. That prior artdevice is provided with an input free beam coupler and an output freebeam coupler the free beam sections of which are coupled to each otherby a light guide grid. Furthermore, the prior art device is providedwith two input light guide units each one of which is provided with aninput light guide. Optical signals of different wavelengths may be fedinto the input light guide of one input light guide unit. This inputlight guide is connected to the input free beam coupler. The input lightguide of the other input light guide unit is connected to the side ofthe output free beam coupler which is connected to the light guide grid.In addition, output light guides of an output light guide unit areconnected to the output free beam coupler, the output light guides beingarranged such that each of them may be energized by a signal from theinput light guide connected to the input free beam coupler and by asignal from the input light guide connected to the output free beamcoupler.

Because of the imaging function of the so-called phased arrayspectograph formed by the input free beam coupler, the light guide gridand the output free beam coupler, the optical signals of differentwavelengths propagating in the input light guide of the input lightguide unit connected to the input free beam coupler are each conductedto associated output light guides. The optical signal propagating in theinput light guide of the input light guide unit connected to the outputfree beam coupler is spreading as a spherical wave into the free beamsection of the output free beam coupler and impinges upon the outputside of the output free beam coupler to which the output light guidesare connected.

While it is possible with the prior art device to carry out the functionof a multiplexer by feeding the wavelengths propagating in the two inputlight guide units to associated output light guides, it suffers from thedisadvantage of resultant relatively high losses.

An optical multiplexer is also known from EP 528,652 which is not,however, provided with any further input light guide unit. Thispublication describes the periodic transfer characteristic which makespossible the occurrence in an output waveguide of signals of wavelengthperiod αλ from an input waveguide (i.e. in addition to wavelength λ₁there occurs the next periodically possible wavelength λ₂=λ₁+Δλ and,further, λ₃=λ₁+2*c16536D Δλ. . . etc.) This constitutes a specificembodiment of a spectograph in planar waveguide technology.

OBJECT OF THE INVENTION

It is an object of the invention to provide a device of the kindreferred to hereinabove which is characterized by low attenuationlosses.

BRIEF SUMMARY OF THE INVENTION

The object is accomplished by the at least one other input light guideunit converging into the free beam section of the input free beamcoupler and by converging sections of the input light guide units beingarranged and dimensioned according to the equation.$\left. {\Theta_{IN}^{(i)} = {\frac{m}{n_{s}d}\left\{ {\lambda^{(i)} - {{\lambda_{c}\left\lbrack {1 + {\frac{1}{n_{c}}\quad \frac{n_{c}}{\lambda}}} \right.}_{\lambda = \lambda_{c}}\left( {\lambda^{(i)} - \lambda_{c}} \right)}} \right\rbrack}} \right\} - \Theta_{OUT}^{(i)}$

wherein λ_((i)) is the wavelength conducted in an input light guide ofthe input light guide units; λ_(c) is the central wavelength of thedevice; m is the order of refraction of the light guide grid; n_(c) isthe effective index of refraction of the phase shift light guide; n_(s)is the effective refractive index of the free beam sections; d is thegrid constant or pitch of the phase shift light guides at the transitionto the output free beam section of the output free beam coupler; ⊖^((i))_(IN) is the incoupling angle of the i^(th) input light guide relativeto the input axis of symmetry and ⊖^((i)) _(OUT) is the outcouplingangle relative to the output axis of symmetry of the output free beamsection, such that each output light guide may be charged with at leasttwo optical signals of different wavelengths which are conducted in atleast two input light guide units.

The central wavelength λ_(c) of the device is derived from the mean oraverage between the broadcast wavelength or the mean wavelength of thespecific wavelengths.

By connecting the input light guide units to the input free beam couplerand by arranging and dimensioning the output sections of the input lightguide units relative to the output light guides such that each outputlight guide may be charged with at least two optical signals ofdifferent wavelengths, the optical signals of the input light guidesections are fed at low attenuation losses and in a controlled manner tothe output light guides by way of the imaging function of the so-calledphased array spectograph formed by the input free beam coupler, thelight guide grid and the output free beam coupler. By separating thepower division which takes place entirely within the input free beamcoupler, from the imaging controlled superposition in the output freebeam coupler, optimizing measures may be separately carried outeffectively at the input free beam coupler and the input light guideunits connected thereto and the output free beam coupler and itsconnected output light guide unit.

In one embodiment of the invention at least a first input light guideunit is provided with a plurality of input light guides connected to theinput free beam coupler. In a related variant, optical signals ofdifferent wavelengths grouped around a center wavelength may beconducted in the individual input light guides of the first input lightguide unit, the input light guides being arranged and dimensionedrelative to the output light guides such that each output light guide isprovided with an associated input light guide of the first input lightguide unit.

In another variant, there is provided a second input light guide unitprovided with individual input light guides which may be charged with anoptical signal of a single wavelength, the input light guides beingarranged and dimensioned relative to the output light guides such thateach output light guide is associated with an input light guide of thesecond input light guide unit.

In an advantageous embodiment, there are provided two input light guidesprovided with input light guides as in the preceding embodiments so thateach output light guide is associated with exactly one input light guideof the first input light guide unit and one input light guide of thesecond light guide unit. This results in a particularly low systemsattenuation at a controlled multiplex function.

In a further embodiment, at least one input light guide unit is providedwith an input light guide which preferably flares out hyperbolically inthe direction of the input free beam coupler. Into this single flaringinput free beam coupler an optical signal of a single wavelength may befed which signal is to be distributed to the output light guides. Whilesuch a structure leads to somewhat higher attenuation losses, it permitscoupling-in so-called broadcast wavelengths which fluctuate relativelystrongly, for the flaring of the input light guide ensures that even atfluctuating wavelengths the output light guides are charged with thecorresponding optical signal.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, in respect of its structure, construction andlay-out as well as manufacturing techniques, together with other objectsand advantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 depicts an embodiment of a device for superposing optical signalsof different wavelengths with input light guide units provided with twoinput light guides;

FIG. 2 shows, on an enlarged scale relative to FIG. 1, a furtherembodiment for superposing optical signals of different wavelengths inwhich an input light guide unit is provided with a plurality of inputlight guides and in which each of two further input light guide units isformed by an input light guide flaring in the direction of the inputsection in a free beam section of an input free beam coupler.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts an embodiment of a device for superposing optical signalsof different wavelengths provided with an input free beam coupler 1. Inthe depicted embodiment, eight input light guides 4, 5, 6, 7, 8, 9, 10,11 of a first input light guide unit 12 are connected by their outputends to a convex circularly curved input side 3 which limits an inputfree beam section 2 of the input free beam coupler 1, the output endsleading into the input free beam section 2 at a first input section. Inaddition, in the depicted embodiment the output ends of eight inputlight guides 13, 14, 15, 16, 17, 18, 19, 20 of a second input lightguide unit 21 are connected to the input side 3, so that the output endsof the light guides lead into the input free beam section 2 at a secondinput section. The input light guide units 12, 21 are arranged at adistance from each other with the optical position of a centerwavelength lying between the input sections.

A plurality of phase shift light guides 23 of a light guide grid 24 areconnected to the convex circularly curved output side 22 of the inputfree beam coupler 1. The phase shift light guides 23 are formed ofdifferent lengths the difference in length between neighboring phaseshift light guides 23 being constant. The constant difference in lengthis derived in a known manner from the order of the grid and the centerwavelength.

The phase shift light guides 23 shown in a shortened state in FIG. 1 areconnected to an input side 25 of an output free beam coupler 26 and leadinto the output free beam section 27 thereof. Eight output light guides29, 30, 31, 32, 33, 34, 35, 36 of an output light guide unit 37 areconnected to a convex circularly shaped output side 28 of the outputfree beam coupler 26.

Preferably, the free beam couplers 1, 26 as well as the light guides 4to 11, 13 to 20, 23, 29 to 36 of the light guide units 12, 21, 37 aswell as the light guide grid 24 are made in planar light guidetechnology.

At their sections leading into the input free beam section 2, the inputlight guides 4 to 11 of the first input light guide unit 12 as well asthe input light guides 13 to 20 of the second input light guide unit 21are structured and dimensioned such that each output light guide 29 to36 of the output light guide unit 37 may be charged with at least twooptical signals of certain wavelengths.

It will be understood that there may be provided a greater as well as alesser number of input light guides 4 to 11, 13 to 20 of the input lightguide units 12, 21 as well as of the output light guides 29 to 36.

In the embodiment shown in FIG. 1 an optical signal of a wavelength,namely of a so-called broadcast wavelength, may be fed into the inputlight guides 13 to 20 of the second input light guide unit 21. Forinstance, the wavelength may be fed by an input light guide to a starcoupler not shown in FIG. 1 for distribution to the input light guides13 to 20 of the second input light guide unit 21. An optical signal maybe fed into each of the input light guides 4 to 11 of the first inputlight guide unit 12; the optical signals fed into the input light guides4 to 11 of the first input light guide unit 12 have a carrier ofdifferent wavelengths. The wavelengths which may be fed into the inputlight guides 4 to 11 of the first input light guide unit 12 arepreferably grouped about a center wavelength which differs from thebroadcast wavelength which may be fed into the input light guides 13 to20 of the second input light guide unit 21, and among each other theyhave a much smaller wavelength spacing compared to the wavelengthspacing between the broadcast wavelength and the center wavelength. Thearrangement and dimensioning of the input sections of the input lightguides 4 to 11 and 13 to 20 relative to the input sections of the outputlight guides 29 to 36 ensures that a specific wavelength and thebroadcast wavelength may be fed into each output light guide 29 to 36.

In this connection, at a given position of the output light guide 29 to36 associated with it, the position of an input light guide 4 to 11 ofthe first input signal light guide unit 12 is determined in accordancewith the following basic positioning equation:$\left. {\Theta_{IN}^{(i)} = {\frac{m}{n_{s}d}\left\{ {\lambda^{(i)} - {{\lambda_{c}\left\lbrack {1 + {\frac{1}{n_{c}}\quad \frac{n_{c}}{\lambda}}} \right.}_{\lambda = \lambda_{c}}\left( {\lambda^{(i)} - \lambda_{c}} \right)}} \right\rbrack}} \right\} - \Theta_{OUT}^{(i)}$

wherein λ^((i)) is the wavelength propagating in an input light guide 4to 11 of the first input light guide unit 12, λ_(c) is the centerwavelength of the device derived from the median between the broadcastwavelength or center wavelength of the specific wavelength, m is theorder of refraction of the light guide grid 24, n_(s) is the effectiverefractive index of the free beam sections 2, 27, d is the grid constantor pitch of the phase shift light guide 23 at its transition to theoutput free beam section 27 of the output free beam coupler 26,⊖^((i))/_(IN) is the incident coupling angle 38 of the i^(th) inputlight guide 4 to 11 relative to the input axis of symmetry 39 of theinput free beam section 2 and ⊖^((i))/_(OUT) is the output angle 40relative to the output axis of symmetry 41 of the output free beamsection 27. In then presentation of FIG. 1 the axes of symmetry 39, 41and the coupling angle 38, 40 have been shown for the first input lightguide 4 and its associated eighth output light guide 36.

When a so-called broadcast wavelength is fed into the input light guides13 to 20 of the second input light guide unit 21 the intermediate anglesbetween neighboring output light guides 29 to 36 correspond to theintermediate angles between associated neighboring input light guides 13to 20 of the second input light guide unit 21.

For purposes of a relatively simple fabrication it is useful to arrangethe output light guides 29 and 36 equidistantly and centered around theaxis of symmetry 41 so that a precision arrangement will only berequired for the input light guides 4 to 11, 13 to 20.

In a variant of the embodiment shown in FIG. 1 the input light guides 13to 20 of the second input light guide unit 21 are arranged relative tothe output light guides 29 to 36 such that optical signals of furtherdifferent specific wavelengths fed into the input light guides 13 to 20of the second light guide input unit 21 may be fed as carriers to eachoutput light guide 29 to 36 which signals will be superimposed on thebroadcast wavelength fed into the input light guides 13 to 20 of thesecond input light guide unit 21. In this variant, the intermediateangles between neighboring output light guides 29 to 36 and intermediateangles between associated neighboring input light guides 13 to 20 of thesecond input light guide unit 21 are set up in accordance with thepositioning equation set forth supra.

In a further variant of the embodiment shown in FIG. 1, the input lightguides 4 to 11 of the first input light guide unit 12 are arranged,relative to the output light guides 29 to 36, such that a furtherbroadcast wavelength fed into the input light guides 4 to 11 may be fedto each output light guide 29 to 36 which will superimpose on thebroadcast wavelength fed into the input light guides 13 to 20 of thesecond input light guide unit 21.

Since frequently a broadcast wavelength is generated with a certaintolerance by an optical transmitter, it is useful to fabricate the inputport sections of the input light guides 4 to 11, 13 to 20 in which anyone of the broadcast wavelengths is propagating in so-called multi-modeinterference technology in accordance with which the input port sectionsare flared out so that owing to the propagation characteristics of thephased array spectograph formed by the input free beam coupler 26, thelight guide grid 24 and the output free beam coupler 26, all outputlight guides 29 to 36 are uniformly energized in a substantiallyrectangular intensity profile. Owing to the broader intensity profilethe wavelength tolerance is markedly increased.

FIG. 2 depicts, in a presentation of enlarged scale relative to FIG. 1,a further embodiment of a device for superposing optical signals ofdifferent wavelengths, in which a first input light guide unit 12structured similarly to the embodiment of FIG. 1, is provided with inputlight guides 4 to 11 which enter into the input free beam coupler 2 atthe input coupling side 3. The embodiment according to FIG. 2 isadditionally provided with a first broadband input light guide unit 42which, being a so-called multi-mode interference light guide, at theinput port section of its input free beam coupler 2 is of a preferablyhyperbolically flaring structure. Furthermore, the embodiment of FIG. 2is provided with a second broadband input light guide unit 44 which isalso provided with a multimode interference light guide 45. In thismanner, an image of a substantially rectangular intensity distributionof any signal which may be fed into this input light guide unit 42, 44may be formed in the input port section of the output light guides 29 to36. In the embodiment depicted in FIG. 2, the input axis of symmetry 39of the input free beam coupler 1 is extending substantially in themiddle between the first input light guide unit 12 and the firstbroadband input light guide unit 42, relative to their respective centeraxes.

A first so-called broadcast wavelength may be fed into the multi-modeinterference light guide 43 of the first broadband input light guideunit 42, and a second so-called broadcast wavelength may be fed into themulti-mode interference light guide 45 of the second broadband inputlight guide unit 44. Preferably, the broadcast wavelengths arespectrally separated sufficiently to separate the input port sections ofthe multi-mode interference light guides 43, 45 from each other. Thisresults in a clean spatial separation and image or aperture on theoutput light guides 29 to 36 of the optical signals which may be fedinto the broadband input light guide units 42, 44. The multi-modeinterference light guides 43, 45 of the broadband input light guideunits 42, 44 are arranged such that output light guides 29 to 36 of theoutput light guide unit 37, not shown in FIG. 2, may receive bothbroadcast wavelengths.

Furthermore, each output light guide 29 to 36 may receive the specificwavelength propagating in one of the input light guides 4 to 11 of thefirst input light guide unit 12 so that a specific wavelength as well asthe two broadcast wavelengths are conducted in each output light guide29 to 36. Because of the relatively large input port section of themulti-mode interference light guides 43, 45 the output light guides 29to 36 may, within a relatively wide range of wavelength tolerances, beenergized substantially uniformly with the broadcast wavelengths.

What is claimed is:
 1. A device for superposing optical signals of different wavelengths, comprising: an input free beam coupler provided with a first free beam section; an output free beam coupler provided with a second free beam section; a light guide grid comprising phase shift light guides for coupling the first and second free beam sections; a first input light guide unit for propagating at least one wavelength and connected to the input free beam coupler; at least one second input light guide unit for propagating at least one wavelength and coupled to the first free beam section; an output light guide unit coupled to the output free beam coupler and comprising a plurality of output light guides; the input sections of the first and at least second input light guide units being arranged and dimensioned relative to an axis of symmetry of the output free beam section in accordance with formula $\left. {\Theta_{IN}^{(i)} = {\frac{m}{n_{s}d}\left\{ {\lambda^{(i)} - {{\lambda_{c}\left\lbrack {1 + {\frac{1}{n_{c}}\quad \frac{n_{c}}{\lambda}}} \right.}_{\lambda = \lambda_{c}}\left( {\lambda^{(i)} - \lambda_{c}} \right)}} \right\rbrack}} \right\} - \Theta_{OUT}^{(i)}$

λ^((i)) being the wavelength propagating in an input waveguide of the input light guide units; λ_(c) being the central wavelength of the device; m being the order of diffraction of the light guide grid; n_(c) being the effective index of refraction of the phase shift light guide; n_(s) being the effective index of refraction of the free beam sections; d being the grid constant (pitch) of the phase shift light guides at the transition to the second output free beam section; ⊖^((i)) _(IN) being the incident coupling angle of the i^(th) input light guide relative to an input axis of symmetry of the first input free beam section and ⊖^((i)) _(OUT) being the output angle of divergence relative to the axis of symmetry of the output free beam section, such that each output light guide is energized by at least two optical signals of different wavelengths propagating in the first and at least second input light guide units.
 2. The device of claim 1, wherein at least three input light guide units are coupled to the input free beam coupler.
 3. The device of claim 1, wherein at least the first input light guide unit comprises a plurality of input light guides, the input light guides and the output light guides being arranged such that upon energizing each input light guide with a signal of a different wavelength each output wave guide is energized by a signal from a light guide of the first input light guide unit and a signal from the at least second input wave guide unit.
 4. The device of claim 1, further provided with a third input light guide unit comprising a plurality of third input light guides, the input light guides of the first input light guide unit and of the third input light guide unit and the output light guides being arranged such that upon energizing each input light guide of the first light guide unit with a signal of a different wavelength and upon energizing the input wave guides of the third input wave guide unit with a signal of a further wavelength each output wave guide is energized by signals from associated input wave guides of the input wave guide units.
 5. The device of claim 1, wherein at least one input wave guide unit is structured by light guides preferably hyperbolically flaring in the direction of the input of the free beam section such that a substantially rectangular distribution of intensity of the signal fed into the input light guide unit may be imaged in the input section of the output light guides.
 6. The device of claim 1, wherein the output light guides are arranged at uniform distances from each other.
 7. The device of claim 1, wherein the input sections of the input light guide units are spaced at a distance from each other.
 8. The device of claim 1, wherein the input light guides and the light guides of the light guide grid are structured as planar light guides.
 9. The device of claim 1, wherein the input light guides of an input light guide unit propagating a signal of a wavelength are connected to the outputs of a star coupler. 